We aim to prevent Alzheimer disease (AD) in DS (trisomy 21) (AD-DS). Using antisense oligonucleotides (ASOs), we will selectively target RNA for the amyloid precursor protein (APP) in mouse models of AD-DS (Dp16) and AD/cerebral amyloidosis (Line 41). The therapeutic premise is based on: 1) increased APP gene dose is necessary for AD-DS. As replicated in models of AD-DS, normalizing APP dose eliminated: a) age-related neurodegeneration in locus coeruleus and the basal forebrain complex, b) hyper- phosphorylation of Tau, and c) enlargement of early endosomes; 2) pointing to a mechanism by which increased APP gene dose acts, increased full-length APP (fl-APP), its 99 residue C-terminal fragment (C99) and A?42 each increased Rab5 activity, thus enlarging early endosomes, disrupting endosomal trafficking of neurotrophic signals, and causing atrophy of BFCNs; 3) therefore, reducing levels of these APP products is a rational approach to preventing or lessening the impact of increased APP gene dose in AD-DS, including effects on endosomes. ASOs have recently been shown to safely and effectively treat CNS disorders. Indeed, FDA approval for ASOs in Spinal Muscular Atrophy motivates trials of ASOs in other CNS diseases. In preliminary studies we showed that intracerebroventricular (ICV) injection of ASOs targeting mouse and human APP (i.e. mAPP-ASOs and hAPP-ASOs) reduced APP mRNA and protein levels. Using mouse models of AD-DS and AD/cerebral amyloidosis we will test the therapeutic hypothesis that APP-ASOs will selectively reduce the levels of APP mRNA and its products to prevent and/or lessen neurodegeneration. The mechanistic hypothesis is that APP-ASOs will normalize endosomal structure and function, neurotrophin signaling and trafficking, and improve cognition. Using defined GO/NOGO criteria as a guide, we will pursue these Specific Aims: 1. To investigate newly designed APP-ASOs in vitro for efficacy and target specificity. Using an existing mAPP-ASO as benchmark, additional mAPP-ASOs will be designed to increase potency for targeting APP mRNA and its products and normalizing endosome size. 2. To establish optimal APP-ASO doses and dose-intervals based on empirically defined in vivo pharmacokinetic (PK) and pharmacodynamic (PD) properties. We will define effective, non-toxic doses and treatment intervals for advancement of mAPP-ASOs and hAPP-ASOs to in vivo studies in Aim 3. In the Dp16 model, we will target a ~33% reduction of mAPP RNA, i.e. to 2N values; in Line 41 mice we will target a 50% reduction. Aim 3. To investigate in vivo APP-ASO efficacy in ameliorating neurodegeneration and normalizing endosomal phenotypes. To test the therapeutic hypothesis, we will ask if APP-ASOs given before degeneration in Dp16 mice and plaque deposition in Line 41 mice prevent these changes. Next, we will ask if degeneration in Dp16 mice can be reversed by APP-ASO treatment. The mechanistic hypothesis will be informed by whether or not APP-ASO reductions in degeneration are correlated with normalization of endosomal phenotypes.